The DLink DSL-2640B

[#1]

A major improvement you can make to wireless is to change the channel from the default (often 6).
To channel 1, 6 or 11 (don't use any other).
I don't know if there is anything for Windows but for Linux enter the command

iwlist scan

It will show the channel used and the signal strength for all nearby access points (and other information).

[#2]

Got the replacement today & it is now set up and working a treat. Wireless signal is pretty decent around the house and I'm getting far higher connection speeds. I know it'll take a day or two to see the full benefit of sync'ing at over a Mbit higher (around 7.5Mbit) but even rendering pages is a lot faster.
Once again thanks to the Plusnet team for getting this sorted and the faulty router will be dispatched back to you asap.